The regulation is one of many Brussels rules, including GM food standards, that could be a significant stumbling block to a future free-trade deal

Mr Ross has been here for talks designed to ensure trans-Atlantic trade and travel continues smoothly after March, 2019. He said the visit let him “address some concerns we have that they may be tempted to include (provisions) in their agreement with the European Commission that could be problems for a subsequent free trade agreement with the US”.

He made it clear the US favours keeping many of Britain’s current arrangements with the EU.

In particular, passporting rights that let UK-based firms offer financial ­services in the other 27 states, which have made London a gateway into Europe for US banks.

Mr Ross called a free trade agreement with the UK a priority for Washington, but said it would be “very complex” to complete.